Dude...
This doesn't have to be complicated or expensive.
Head to Dell's Website or eBuyer! Grab yourself a entry level server.
Make sure you have some type of RAID there for redundancy. Also grab yourself a cheap NAS.
In my opinion you could slash your costs whacking CentOS 5.5 on there with Samba Server to do the Windows Domain/File Sharing role.
Just incase you don't know what CentOS is, it's pretty much Redhat Enterprise Linux 5 but community supported.
There are plenty of tutorials around for setting up CentOS to do all sorts.
If you need help though just post on here or PM me.
Cheers
Matt